1. Field of the Invention
This invention relates to a method of burning pulverous raw material containing lime, such as cement raw meal. The invention also relates to a rotary kiln plant for burning such materials.
2. Description of the Prior Art
Calcination of pulverous raw materials such as cement raw meal is to be understood as an expulsion of carbon dioxide (CO.sub.2) from calcium carbonate by an endothermic process (i.e. a process in which heat is absorbed) according to the equation: EQU CaCO.sub.3 .fwdarw. CaO + CO.sub.2
when the raw material is cement raw meal, the aforesaid finishing heat treatment following the calcination is a sintering by which cement clinker is produced. Sintering is an exothermic process characterized by, or formed with, evolution of heat. The sintering therefore only requires a modest supply of heat in order to raise the temperature of the raw material to the sintering temperature and to compensate for losses.
The heat necessary for carrying through the conversion of cement raw meal to cement clinker is usually obtained by burning fuel which together with combustion air, is introduced into a combustion chamber in which the fuel burns successively with the combustion air and forms smoke gas. As a result, the energy contained in the fuel is released for heating the smoke gas to a high temperature. The hot smoke gas is then brought into contact with the raw meal to be heat treated, i.e. preheated, calcined and burned. The heat treatment usually takes place as a continuous process in a rotary kiln with slightly inclining axis. Thus, the lower lying end of the rotary kiln is - as is usual for rotary kilns - designed as a combustion chamber.
However, in the manufacture of cement the trend is to carry through preheating and calcination of the raw meal separately by means of hot gas from one source of heat and sintering of the raw meal in a rotary kiln by means of hot gas from another source of heat, that is the flame of the rotary kiln. This is particularly due to the presence of alkalies in the raw meal.
As far as the calcination of cement raw meal is concerned, relatively large quantities of materials must be subjected to a relatively low heating. That is, the process takes place at a rather low temperature which is often inconsistent with the use of smoke gases of a high temperature for carrying through the calcination. There is a great risk of excessively heating the raw meal locally and temporarily. Even a brief period of excessively heating a part of the raw meal may cause expulsion of alkali vapors or produce melts which may involve caking problems. Furthermore, excessive heating of the raw meal at the calcination stage may prevent chemical reactions intended for a later stage of the whole process of manufacturing cement clinker. For example, the formation of clinker minerals at such stage of the whole heat treatment process at which calcination is to take place involves an unfavorable and disadvantageous development of the whole process.
U.S. Pat. No. 3,203,681 to Rosa et al. relates to a process wherein heat for carrying through the calcination of preheated cement raw meal derives from hot gases having a temperature higher than the calcination temperature. The gases are produced in a separate chamber and are passed upwardly in a riser column in which the raw material is suspended and entrained by the gases thus produced.
In a related development, commonly assigned U.S. patent application Ser. No. 423,436, filed December 10, 1973, now U.S. Pat. No. 3,955,995 is directed to calcination of pulverous material by mixing preheated raw material intimately with a fuel, capable of producing a combustible gas upon contacting the raw material, providing a gas capable of supporting combustion of the combustible gas thus produced to at least partially calcine the raw material, and separating the treated raw material from the stream of gases. U.S. patent application Ser. No. 450,291, filed Mar. 12, 1974 relates to a method of heat treating a preheated pulverous raw material wherein the material is accumulated in a lower portion of a calcination chamber and an oxygen-containing gas is introduced into a flame overlying the accumulation. I have invented a method of burning pulverous raw materials and a rotary kiln plant therefor wherein the raw material is at least partially calcined in an efficient manner prior to burning it in a rotary kiln by utilizing a portion of the rotary kiln and the tumbling action provided thereby for mixing preheated raw material with a fuel capable of producing a combustible gas upon contacting the raw material.